A requirement of magnetically-locked exit doors is that the magnetic lock be deactivatable from within a building upon demand by a user desiring egress. A typical exit door is provided with a horizontal electromagnetic lock-deactivating bar mounted across the inner surface of the door and responsive in any of a wide variety of ways to pressure by a user. In the art, it is considered to be good design to provide two or more redundant deactivating systems to ensure that a door may be opened even if one of the systems malfunctions.
For example, U.S. Pat. No. 4,871,204 discloses a release system comprising a capacitive circuit, including a relay, that senses any touching of a fixed horizontal bar in a first user action, and a separate back-up micro-switch activatable circuit. The capacitive circuit relay, when engaged in response to the human touch sensor, includes means for opening the locking circuit for the electromagnetic lock. In the event of malfunction of the capacitance sensor system, egress can still be accomplished, in a second and separate user action. This is accomplished by pressing the push button switch, mounted on or near the bar, which activates the capacitive circuit relay.
A shortcoming of the disclosed system is that the capacitive sensor output signal can be corrupted by electronic noise, causing the door to become unlocked when not intended and potentially allowing ingress from the outside. A further shortcoming is that a second distinct user action is required to open the door if the capacitive sensor system fails. A user may not know of the push button switch, or a user may not remember the position of the push button switch or how the switch may be activated, especially in this system since the switch is concealed behind the push bar.
For another example, U.S. Pat. No. 5,969,440 discloses a release system comprising two electromechanical force transducer assemblies mounted within a moveable bar and responsive to translation of the bar. When a given amount of pressure is detected by either or both of the electromechanical force transducer assemblies, the door will be unlocked and can be subsequently opened. A back up switch is also located on the bar and will operate in a fail-safe manner (without power) to unlock the door in the event of a failure of one or both of the transducer assemblies upon detection of a greater amount of force being exerted upon the bar.
A shortcoming of the disclosed system is that activation of either or both of the transducers requires substantial force, for example, between 5 and 15 pounds of pressure, and operation of the back-up micro-switch requires not less than 15 pounds of pressure. The large force required to operate the back-up switch is needed to ensure that the pressure transducers come into play before the back-up switch is used. Forces in this high range may be beyond the capabilities of a user in a given situation. Further, the disclosed electromechanical force sensors use force sensing resistors whose sensitivity and output may change with aging of the sensors or of the associated actuating padding material.
In yet another example, U.S. Pat. No. 6,429,782 B2 discloses a door release system comprising a conductor forming part of a capacitor with variable capacitance dependent upon the proximity of a person, and a detector for sensing variation in the capacitance and for generating an output signal indicating proximity of the person relative to the conductor as the person's hand touches the release. The switch device further includes a mechanical switch arranged for actuation by a person gripping or pulling the door handle to additionally or alternatively indicate proximity of the person. The system includes an oscillator coupled to the conductor and a phase comparator. The variation in capacitance results in an associated change of frequency in the oscillator to produce a phase-modulated signal which is applied to the phase comparator to generate a signal representative of the change in frequency.
What is needed in the art is a door latch release system that includes a capacitive circuit including at least one capacitive sensor, a micro-processor programmed with noise-discrimination software to sense touching of the bar and a micro-switch, or switches, as back-up that picks up movement of the bar to release the door should the capacitive circuit be unresponsive.
It is a principal object of the present invention to increase the reliability of a door is release system by incorporating a back-up system that can redundantly release the door upon a natural and continued motion of the person opening the door.
It is a further object of the present invention to increase the reliability of a door release system by discriminating against spurious noise signals that can cause a capacitive switching system to open inadvertently or to be opened maliciously while also providing a back-up door release switch which is less sensitive to pounding on the exterior side of the door.